Engineers in open hut worried at that point (9)
I believe the answer is:
thereupon
'at that point' is the definition.
(I've seen this before)
'engineers in open hut worried' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'in' means one lot of letters goes inside another.
'worried' is an anagram indicator.
'open'+'hut'='openhut'
'openhut' with letters rearranged gives 'theupon'.
're' inserted inside 'theupon' is 'THEREUPON'.
(Other definitions for thereupon that I've seen before include "straight afterwards" , "Shortly after that" , "at that point" , "immediately after that" , "straightaway" .)